Oscillators are commonly used in a variety of communication systems, such as radio frequency (RF) systems and other wireless communication systems. Within the variety of communication systems, oscillators are typically used in transmitter and receiver circuits of the communication systems. A common type of oscillator is the voltage-controlled oscillator (VCO).
One typical component of the VCO is a bipolar transistor (BJT). Another suitable component for the VCO is a complimentary metal oxide semiconductor (CMOS) transistor, which has lower power consumption than the BJT. However, a VCO implemented with CMOS transistors is inherently limited to operable frequencies of no more than 10 gigahertz (GHz). This is because as the frequency of the VCO implemented with CMOS increases to around 10 GHz, important operating characteristics of the VCO, for example phase-noise and differential output deteriorate increasingly. The deterioration of the differential output has an adverse effect on the quality of frequency generation by the VCO. The generated frequency is directly provided to a mixer's local oscillator input or a power amplifier input, essentially causes undesirable effects during transmission and reception of radiated signals. Therefore, the differential output of a VCO is an important consideration when designing the VCO.
An example of a commonly used VCO is the cross-coupled inductor capacitor (LC) voltage-controlled oscillator 100 implemented with a differential buffer 102 as shown in FIG. 1. The cross-coupled LC voltage-controlled oscillator 100 is typically coupled to the differential buffer 102 for differentiating the single-ended outputs of the cross-coupled LC voltage-controlled oscillator 100. The differential output generated by the differential buffer 102 is usually directly fed to the mixer's local oscillator. The oscillating frequency of the cross-coupled LC voltage-controlled oscillator 100 is dependent on the capacitance of both varactors Dv, the inductance of inductor L1 and accompanying parasitic capacitance and inductance associated with the varactor Dv and inductor L1 respectively. The differential buffer 102 has two inductors L2 and L3 for providing the differential output through output terminals Out_P, Out_M typically required by the mixer's input. A larger physical area of the differential buffer 102 and higher current are required when the cross-coupled LC voltage-controlled oscillator 100 is used for operating at higher oscillating frequency. This inevitably causes an increase in chip area and requires more current to operate at higher operating frequency. Implementation cost would be higher due to the number and physical size of circuit components used.
FIG. 2 shows a schematic diagram of a Colpitts oscillator 200 implemented with a single-ended-to-differential converter 202. The Colpitts oscillator 200 is commonly known to generate single-end output. The oscillating frequency of the Colpitts oscillator 200 is dependent on inductor L1, capacitors C1 and C2 and associated parasitic inductance and capacitance. Ideally and more specifically, the frequency of oscillation fo of the Colpitts oscillator 200 is dependent on the inductance L1 of inductor L1, the capacitances C1, C2 of capacitors C1 and C2 respectively and is represent by the following relationship:
  fo  =      1          2      ⁢      π      ⁢                        L1          ⁡                      (                          C1C2                              C1                +                C2                                      )                              
The frequency of oscillation fo is therefore controllable by varying capacitances C1, C2. The values of capacitances C1, C2 may be voltage-controlled when both capacitances C1, C2 are adjustable by the voltages across each capacitor C1 and C2. Hence, the Colpitts oscillator 200 may be used as a VCO by making capacitors C1 and C2 capacitance adjustable.
During actual operation of the Colpitts oscillator 200, the associated parasitic inductance and capacitance of the Colpitts oscillator 200 become more prominent as the frequency of oscillation fo increases. This renders the Colpitts oscillator 200 unusable to operate at high frequency, such as an operating frequency of 10 GHz.
The single-ended-to-differential converter 202 that is coupled to the Colpitts oscillator 200 performs a similar function as the differential buffer 102 of the cross-coupled LC voltage-controlled oscillator 100 of FIG. 1. The single-ended-to-differential converter 202 can be active or passive and converts the single-end outputs of the Colpitts oscillator 200 to differential output. A passive single-ended-to-differential converter has high power loss and is therefore not suitable for use in systems that have low thermal heat tolerance. An active single-ended-to-differential converter usually needs higher power for operation as operating frequency increases and is therefore not desired for use in systems that require low power consumption.
There is therefore a need for a voltage-controlled oscillator with low current consumption and which is capable of performing high frequency operation and provides differential output.